The use of multiple transmit/receive antennas in wireless networks promises mitigation of interference and improved spectral efficiencies through focusing signals along a designated direction (“beamforming,” “directional beamforming,” or “directional focusing”), or on an intended receiver (“location-focusing” or “spot-focusing”). Compared to single-antenna-to-single-antenna transmissions, transmit beamforming may yield increased range (e.g., an N-fold increase for free-space propagation), increased rate (e.g., an N2-fold increase in a power-limited regime), increased power efficiency (e.g., an N-fold decrease in the net transmitted power for a fixed received power), and/or may allow splitting a high data-rate stream into multiple lower data-rate streams. (Here, N is the number of cooperative nodes or antenna elements at the transmit side.)
Distributed or decentralized coherent RF transmit beamforming or spot-focusing is a form of cooperative communication in which two or more nodes (that is, nodes of an array of nodes) simultaneously transmit a common message, controlling the phase and timing of their transmissions so that the transmitted signals constructively combine at an intended destination or direction.
In the case of directional beamforming, the individual array nodes may be configured as a phased array to produce a beam that is approximately collimated in a given direction, but the beam is not specifically focused to maximize power at a given location of the target receiver. Phased arrays where the locations of the individual array elements and the target receiver are known, where the array elements are interconnected with cables or other calibrated interconnections (e.g., hardwired), and where a common centralized clock/time reference can be distributed among the array elements, can be configured to operate in such directional beamforming modes.
Decentralized or distributed arrays may be much more difficult to use as coherent beamforming phased arrays, either in transmit mode or receive mode. In a decentralized array, the individual nodes are untethered devices with independent clocks, i.e., without a distributed/hardwired clock or frequency reference. Additionally, in a decentralized array the precise positional coordinates of each node may be unknown and/or varying in time. Decentralized cooperative arrays and their operation for radio frequency (RF) communications are described in several commonly-owned and related patent documents, including the patent applications listed under the Cross-Reference to Related Applications heading and the following:
1. International Patent Publication WO/2012/151316 (PCT/US2012/36180), entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL, filed 2 May 2012;
2. U.S. patent application Ser. No. 14/114,901, Publication Number 2014-0126567, entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL, filed on 30 Oct. 2013;
3. U.S. Provisional Patent Application Ser. No. 61/481,720, entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL FOR COMMUNICATIONS, SENSING & IMAGING, filed on 2 May 2011;
4. U.S. Provisional Patent Application Ser. No. 61/540,307, entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL FOR COMMUNICATIONS, SENSING & IMAGING, filed on 28 Sep. 2011;
5. U.S. Provisional Patent Application Ser. No. 61/809,370, entitled APPARATUS, METHODS, AND ARTICLES OF MANUFACTURE FOR COLLABORATIVE BEAMFOCUSING OF RADIO FREQUENCY EMISSIONS OF RADIO FREQUENCY EMISSIONS, filed on 7 Apr. 2013;
6. U.S. Provisional Patent Application Ser. No. 61/829,208, entitled APPARATUS, METHODS, AND ARTICLES OF MANUFACTURE FOR COLLABORATIVE BEAMFOCUSING OF RADIO FREQUENCY EMISSIONS, filed on 30 May 2013;
7. International Patent Application PCT/US2014/033234, entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL, filed 7 Apr. 2014; and
8. U.S. patent application Ser. No. 14/247,229, entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL, filed on 7 Apr. 2014.
Each of the patent documents described above is hereby incorporated by reference, including specification, claims, figures, tables, and all other matter. We may refer to these documents collectively as “incorporated applications” or “related patent documents.”
Several tasks may be necessary or desirable for a decentralized cooperative array of nodes to operate as a directional beamforming or spot-focusing array. First, a decentralized array may need to acquire channel information between the individual array nodes and the intended target/source of transmission, and provide a mechanism for the nodes to transmit/receive a correctly-weighted signal at each of the array nodes (or “elements,” or “members,” which terms are used interchangeably), so that directional beamforming or spot-focusing is achieved to within some predetermined or variable accuracy required by the system's specification or applications.
Second, the information to be transmitted by the decentralized array to a target may need to be distributed across the array (i.e., to the individual nodes). Alternatively and/or additionally, when the array is used for receiving transmissions, the data may need to be collected from the different nodes of the decentralized array.
Third, some control operations may need to be performed across the array.
Fourth, the individual nodes of the decentralized array should be phase-, frequency-, and time-aligned/synchronized, to enable the array to operate in a coherent manner. Frequency alignment/synchronization means that that the nodes can operate on a common frequency. For example, the local oscillators (LOs) of the nodes may all be aligned so that the same frequency ω1 (which may be the frequency of the clock reference of the node or a frequency derived from the clock, such as by multiplication, division, phase lock, frequency synthesis) within some predetermined frequency tolerance is available at each of the nodes. The nodes may then transmit a signal at ω1 by upconverting a baseband signal using the ω1; and can process received signals at ω0 by downconverting them using the locally-generated ω1 or ω0 or yet another downconversion frequency. Time and phase alignment/synchronization means that the nodes agree regarding time/phase reference, within some predetermined time and phase tolerances. Achieving and maintaining alignment/synchronization and coordination of the array nodes is important to the correct operation of the array.
“Homodyne” arrays operate so that ω1=ω0, requiring alignment/synchronization to the target. “Heterodyne” arrays operate so that ω1≠ω0. Heterodyne arrays need not synchronize to the target, but still need to perform “local” alignment/synchronization, that is, alignment of the nodes of the array to some common reference, typically a reference provided by one of the nodes. A heterodyne array may also operate with ω1=ω0 as a special case, but it need not synchronize to ω0.
Some inter-nodal communications (communications between one array node to one or more other array nodes) are needed in such systems. The requirements applicable to the procedures used in the inter-nodal communications may be rather strict, especially those that are imposed by the need to achieve and maintain alignment/synchronization of the different nodes.
The process of alignment/synchronization may need to be repeated, because even when clocks have been aligned, every clock experiences limits as to how long that alignment persists due to random jumps in phase and frequency. This is known as the clock coherence limit. Exceeding the clock coherence limit may manifest as a random scrambling of the phases of the carrier waves used in the directional beamforming or spot-focusing, and a failure to achieve optimal or even minimally-acceptable performance. Even with atomic clocks and with fixed locations of the nodes, the coherence limit is eventually reached, requiring re-alignment of the clocks. In sum, a method used for alignment/synchronization should be fast enough to maintain the alignment required for acceptable communication operation of the array, given the coherence specifications of the clocks of the individual nodes. Moreover, there are other factors that may shorten the time between successive re-alignments, such as the movement of the nodes and the dynamic changes in the channel responses.
Improved techniques for communications between and among nodes are desirable, including improved techniques for time-phase-, and/or frequency-aligning/synchronizing the nodes and maintaining their alignment/synchronization in dynamic environments. Thus, needs exist in the art for improved node-to-node communication techniques for distributed coherent communications between an array of nodes and communication apparatus external to the array; for apparatus, methods, and articles of manufacture enabling such improved communications; and for phase/time/frequency alignment/synchronization techniques that can be used in ad hoc nodes of a distributed array for coherent communications.